1. Field of the Invention
The present invention is in the field of aircraft control systems and more specifically relates to a trimmable feel system for use in conjunction with a power-assisted control system.
2. The Prior Art
Prior to the development of power-assisted control systems for aircraft, the pilot could gauge the amount of control he was applying by the amount of force required to maintain a particular deflection of an aerodynamic control surface. This force was felt by the pilot as he manipulated the control stick. The development of power-aided control presented a problem in that the reaction of the aerodynamic control surface was not transmitted to the pilot's control stick, the reaction forces being overcome by the powered actuators. In response to this problem artificial feel systems were developed so that a reaction force could be felt on the control stick by the pilot as he operated the controls.
The simplest artificial feel system conceivable would consist of a spring loaded mounting for the control stick, such that when the stick is moved from a preselected neutral position, a restoring force is developed in a direction tending to return the control stick to the neutral position.
Such a system is satisfactory when the average position of the control stick coincides with the predetermined neutral position. However, if the control stick has to be maintained for a long period of time at a position other than the predetermined neutral position, it becomes necessary for the pilot to exert a force on the control stick for an extended period of time simply to maintain a chosen setting of the control. The desirability soon became apparent of being able to reset the neutral position of the control stick to coincide with the position found to be used most of the time. The process of resetting the neutral position is known as trimming the feel system.
When the control stick is moved from its neutral position, a force is applied to the control stick linkage by the restoring springs. One prior art system employed a brake connecting the control stick with the portion of the linkage on by the restoring springs. The control stick was moved to a desired new position and then the brake was disengaged. The released portion of the linkage, impelled by the spring force, sped towards the equilibrium spring position, developing sufficient momentum to overshoot the desired position and to oscillate about the desired position until the oscillation died out. If the brake were engaged during this oscillatory phase, a startling and disagreeable transient force was applied to the pilot's control stick.
It was undesirable for the pilot to have to wait for the oscillation to die out before engaging the brake because of the time required. Another approach to the oscillation problem was to critically damp the oscillation. This latter approach required the use of a damper which added weight and complexity to the system.
The present invention overcomes the oscillation problem without the need for adding a damper.